The overall objective for a jet is to convert stored chemical energy in the form of fuel into thrust to propel an aircraft. Early jet engines operate by compressing inlet flow to a high pressure and adding and combusting fuel to achieve a high temperature gas, which then expands across a high-pressure turbine, the power from which is used to drive the compressor. Excess pressure at the turbine exit is used to accelerate the core flow through an exhaust nozzle, thereby achieving high velocities to propel the jet-powered aircraft. Even though early jet engine designs were relatively inefficient, the exhaust velocities were well matched to many jet early aircraft needs.
Turbine temperature limits have continued to increase thereby improving the thermal efficiency of the jet engine core and resulting in higher core turbine exit pressures. However for a turbojet configuration the resulting nozzle exhaust velocities are no longer well matched to most jet aircraft applications, resulting in reduced propulsive efficiency. To overcome these problems turbofan jet engines were developed whereby a power turbine is configured on a separate spool downstream of the core turbine, and the power extracted is used to drive a fan, most of the flow from which bypasses the engine core. The resulting lower velocities from the core and fan nozzles are well matched to most jet aircraft application, resulting in high propulsive efficiency. The combination of high core thermal efficiency and the high turbofan propulsive efficiency have enabled numerous advanced aircraft configurations.
Current turbofan configurations, however, have some drawbacks. Because the fan is generally configured upstream of the core and the power turbine is downstream of the core, an additional shaft must connect the two, which adds additional cost and weight to the engine. Also, the power turbine, which is typically manufactured from high-density nickel super-alloys and is very large relative to the core turbine, adds considerable cost, weight and size to the engine. The added weight, of the additional shaft and power turbine tends to reduce the engine thrust-to-weight ratio. Therefore, while conventional turbofans have proven attractive for many jet aircraft applications, such configurations have not proven attractive as lift engines because of the added cost, weight and size.
Accordingly, a need exists for a turbofan engine with an effective drive mechanism to link the high speed core compressor with the low speed fen, which would improve the engine thrust-to-weight ratio and which would be less costly to manufacture than a traditional turbofan engine.